Double pass interferometer with tilted mirrors

ABSTRACT

An interferometer of the present invention includes a PBS 2  which splits light into reference light and measurement light, a reference mirror  4   a  which reflects the reference light entering the reference mirror from a first direction, a measurement mirror  4   b  which reflects the measurement light entering the measurement mirror from a second direction, a lens system  6  which reflected lights from the reference mirror  4   a  and the measurement light  4   b  enter, a reflective device  5  which reflects light from the lens system  6 , and a light receiving device  16  which receives multiplexed light, wherein the reference mirror  4   a  and the measurement mirror  4   b  are in a conjugate relation with respect to the reflective device  5 , and at least one of the reference mirror  4   a  and the measurement mirror  4   b  is tilted so that its normal direction differs from the first and the second direction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an interferometer, and moreparticularly to a double pass interferometer which obtains displacementinformation of an object to be measured from lights which have beenreflected twice on a reference mirror and a measurement mirror,respectively.

2. Description of the Related Art

Conventionally, as an apparatus which measures a displacement of anobject, controls a stage, or performs a various kind of lengthmeasurements, a laser interferometer has been used because of thefeatures of the high accuracy and high resolution. For example, JapanesePatent Laid-Open No. 2006-112974 discloses a position detectingapparatus which detects a position displacement of an object using aninterference length measurement by non-contact.

FIG. 3 is a configuration diagram of a conventional interferometer. Alaser beam 110 having a wavelength of λ (λ=633 nm) emitted from a lightsource 10 enters a PBS 20 (a polarizing beam splitter) and is split intoreference light 120 a and measurement light 120 b on the PBS surface 20p. The reference light 120 a is reflected on the reference mirror 40 aand enters the PBS 20 again by passing through the previous opticalpath. In this case, A P wave is converted to an S wave by beingtransmitted through a ¼ λ plate 30 a twice. Therefore, it is transmittedthrough the PBS surface 20 p to be reference light 130 a and enter areflective device 50.

On the other hand, the measurement light 120 b is reflected on ameasurement mirror 40 b and passes through a previous optical path toenter the PBS 20 again. In this case, because a light beam of themeasurement light 120 b is transmitted through a ¼ λ plate 30 b twiceand an S wave is converted to a P wave, it is reflected on the PBSsurface 20 p to be a light beam 130 b and, similarly to the referencelight 130 a, enter the reflective device 50.

After that, the reference light 130 a is transmitted through the PBS 20again to be reference light 140 a, and the measurement light 130 b isreflected on the PBS 20 again to be measurement light 140 b. Thereference light 140 a and the measurement light 140 b are transmittedthrough the ¼ λ plates 30 a and 30 b twice, respectively. The referencelight 140 a and the measurement light 140 b entered the PBS 20 again aremultiplexed to be multiplexed light 150. An interference signal having aperiod of ¼ λ in accordance with a displacement of the measurementmirror 40 b can be obtained by receiving the multiplexed light 150 by alight receiving device 160.

As shown in FIG. 3, in a conventional typical interferometer, there area lot of reflective surfaces in the optical path. A component reflectedon an interface 210 b passes through the same optical path as that ofthe ordinary measurement light and is finally superimposed on themultiplexed light 150. Although this reflective component is modulatedby the movement of the measurement mirror 40 b, it reaches themeasurement mirror 40 b by only one reflection. The same is true forinterfaces 210 a, 210 aa, and 210 bb. Therefore, a modulation amount isa half of an ordinary reflective component and is obtained as aninterference signal (a ghost light signal) having a period of ½ λ.

For example, when a reflectance of an AR coat (an antireflective coatingfilm) on the interface 210 b is 0.2%, the interference signal generatedby the ghost light which has been reflected on the interface 210 b hasan interference intensity of no less than around 9%, comparedwave-optically to the interference intensity of a primary signal. Evenif an ultralow reflective AR coat having a reflectance of 0.01% isadopted, the interference intensity is 2.5%.

FIG. 4A is a waveform of an ideal interference signal, and FIG. 4B is awaveform of an interference signal on which a ½ λ periodic error issuperimposed. The interference signal shown in FIG. 4B is a periodicsignal including an error caused by ghost light of a double passinterferometer.

On the electric signal outputted from the light receiving device 160, asine wave signal caused by all ghost lights is superimposed. Therefore,the interference signal obtained by the conventional interferometer hasa waveform as shown in FIG. 4B.

FIG. 5 is a diagram showing a relationship between an output of a lightreceiving device (a sensor displacement output) and a displacement of anobject to be measured.

Commonly, sub-nanometer resolution can be obtained by electricallydividing the sine wave periodic signal modulated in accordance with thedisplacement of the object to be measured. However, if a componentcaused by the ghost light is superimposed, as shown in FIG. 5, thelinearity of the sensor displacement output with respect to thedisplacement of the object to be measured is deteriorated. In otherwords, an interpolation error is included between the sensordisplacement output and the displacement of the object to be measured.Because this error amount reaches several nanometers to tens ofnanometers, it is a big problem in the interferometer used for theapplication requiring ultrahigh accuracy

BRIEF SUMMARY OF THE INVENTION

The present invention provides a high-accuracy interferometer whichsuppresses an effect of ghost light.

An interferometer as one aspect of the present invention includes alight splitting device configured to split light from a light sourceapparatus into reference light and measurement light, a reference mirrorconfigured to reflect the reference light which has been split by thelight splitting device and enters the reference mirror from a firstdirection, a measurement mirror configured to reflect the measurementlight which has been split by the light splitting device and enters themeasurement mirror from a second direction, an optical system configuredso that reflected lights from the reference mirror and the measurementmirror enters the optical system via the light splitting device, areflective device configured to reflect lights from the optical systemin order to irradiate corresponding one of the reflected lights oncorresponding one of the reference mirror and the measurement mirroragain, respectively, and a light receiving device configured to receivelights which have been multiplexed after irradiating corresponding oneof lights twice on corresponding one of the reference mirror and themeasurement mirror, respectively. The reference mirror and themeasurement mirror are in a conjugate relation with respect to thereflective device, and at least one of the reference mirror and themeasurement mirror is tilted so that a normal direction of at least oneof the reference mirror and the measurement mirror differs from thefirst and the second direction.

Further features and aspects of the present invention will becomeapparent from the following description of exemplary embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top view showing a configuration of an interferometer inthe present embodiment.

FIG. 1B is an elevation view showing a configuration of aninterferometer in the present embodiment.

FIG. 2 is one example of an interference pattern which appears in alight receiving area of a light receiving device in the presentembodiment.

FIG. 3 is a configuration diagram of a conventional double passinterferometer.

FIG. 4A is a waveform of an ideal interference signal.

FIG. 4B is a waveform of an interference signal on which a ½λ periodicerror is superimposed.

FIG. 5 is a diagram showing the relationship between a sensordisplacement output and the displacement of an object to be measured ina conventional double pass interferometer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention will be described belowwith reference to the accompanied drawings. In each of the drawings, thesame elements will be denoted by the same reference numerals and theduplicate descriptions thereof will be omitted.

An interferometer of the present embodiment detects displacementinformation of an object (an object to be measured) as phase informationof light, and it obtains the displacement information of the object byconverting the phase information of the light into an electric signal.

FIG. 1A is a top view showing a configuration of an interferometer inthe present embodiment. FIG. 1B is an elevation view showing aconfiguration of the interferometer in the present embodiment.

Reference numeral 1 denotes a light source (a light source apparatus).The light source 1 emits laser light 11 (collimated light beam) whichhas a wavelength of λ (λ=633 nm). The wavelength λ of the laser light 11is not limited to this, but the light source may emit a laser lightwhich has a different wavelength.

Reference numeral 2 denotes a PBS (a polarizing beam splitter). The PBS2 is a light splitting device which splits a light beam into two beamsin accordance with a polarization component of incident light. In thetop view of FIG. 1A, the laser light 11 emitted from the light source 1enters the PBS 2 at an angle α, for example α=5°. The PBS 2 has a PBSsurface 2 p (a polarizing beam splitter surface) and the incident lighton the PBS 2 is split into reference light 12 a and measurement light 12b on the PBS surface 2 p. The top view of FIG. 1A views a plane of thePBS surface 2 a, and the elevation view of FIG. 1B views a cross sectionof the PBS surface 2 p.

Reference numerals 4 a and 4 b denote a reference mirror and ameasurement mirror, respectively. Partial light of the incident lightfrom the light source 1 to the PBS 2 is reflected in a first directionon the PBS surface 2 p, and enters the reference mirror 4 a as areference light 12 a. The other partial light of the incident light tothe PBS 2 is transmitted through the PBS surface 2 p, and enters themeasurement mirror 4 b from a second direction as a measurement light 12b. As shown in FIG. 1B, the reference mirror 4 a reflects the referencelight 12 a which is split by the PBS 2 and enters from the firstdirection. Similarly, the measurement mirror 4 b reflects themeasurement light 12 b which is split by the PBS 2 and enters from thesecond direction. The measurement mirror 4 b is attached to an object tobe measured. When the measurement mirror 4 b (the object to be measured)is displaced in an optical axis direction (the second direction), thephase information of the measurement light 12 b changes. The referencemirror 4 a and the measurement mirror 4 b are in a conjugate relationwith a reflective device 5 described later.

The reference mirror 4 a is arranged with a tilt at an angle θ so that anormal direction of its mirror surface differs from an optical axisdirection (a first direction) of the reference light 12 a reflected onthe PBS surface 2 p. Similarly, the measurement mirror 4 b is arrangedwith a tilt at an angle θ so that a normal direction of its mirrorsurface differs from an optical axis direction (a second direction) ofthe measurement light 12 b transmitted through the PBS surface 2 p. Inthe present embodiment, for example the angle θ is set to 0.5°. In thedescription of the present embodiment, when viewed from the elevationview of FIG. 1B, the angle θ is an angle between the first direction andthe normal direction of the reference mirror surface, and an anglebetween the second direction and the normal direction of the measurementmirror surface.

Therefore, reference light 13 a after reflected on the reference mirror4 a is a light beam with a tilt at an angle 2θ with respect to thereference light 12 a before entering the reference mirror 4 a.Similarly, measurement light 13 b after reflected on the measurementmirror 4 b is a light beam with a tilt at an angle 2θ with respect tothe measurement light 12 b before entering the measurement mirror 4 b.

Reference numeral 6 denotes a lens system (an optical system). Thereference light 13 a reflected on the reference mirror 4 a is reflectedon the PBS surface 2 p and enters the lens system 6 at an angle 2θ.Similarly, the measurement light 13 b reflected on the reference mirror4 b is transmitted through the PBS surface 2 p and enters the lenssystem 6 at an angle 2θ. Thus, the lens system 6 (the optical system) isconfigured so that the reflected light (the reference light 13 a) fromthe reference mirror 4 a and the reflected light (the measurement light13 b) from the measurement mirror 4 b enter it via the PBS 2. As shownin the elevation view of FIG. 1B, the lens system 6 deflects thereference light 13 a and the measurement light 13 b which have enteredit at the angle 2θ to an original angle, i.e. a direction parallel tothe direction of the laser light 11 from the light source 1.

In the present embodiment, the lens system 6 is used as an opticalsystem that the reference light 13 a and the measurement light 13 benter, but the embodiment is not limited to this. Instead of the lenssystem 6, a reflective system can also be used.

In the embodiment, the reference mirror 4 a and the measurement mirror 4b are positioned so that each passing position of the reflected lightsin the lens system 6 is symmetric with respect to a central axis of thelens system 6. In other words, as shown in FIG. 1B, both of thereference mirror 4 a and the measurement mirror 4 b are tilted at anangle θ in a clockwise direction from reference positions, respectively,and the height of each light beam of the reference light 13 a and themeasurement light 13 b which enter the lens system 6 is configured to besymmetric between top and bottom.

The present embodiment is not to this, but the angle of the referencemirror 4 a may be set to an angle differing from that of the measurementmirror 4 b or these mirrors may be tilted in different directions fromeach other. In the embodiment, at least one of the reference mirror 4 aand the measurement mirror 4 b has only to be tilted. In this case, atleast one of the normal directions of the reference mirror 4 a and themeasurement mirror 4 b is tilted with respect to the first direction andthe second direction.

Reference numeral 5 denotes a reflective device. The reference light 13a and the measurement light 13 b which have entered the lens system 6 atan angle 2θ are, as shown in the elevation view of FIG. 1B, refracted bythe lens system 6 to be parallel to each other. The reflective device 5reflects the lights which is parallel to each other from the lens system6 in order to irradiate the reflected lights from the reference mirror 4a and the measurement mirror 4 b (the reference light 13 a and themeasurement light 13 b) on the reference mirror 4 a and the measurementmirror 4 b again.

The reflective device 5 is in a conjugate relation with the referencemirror 4 a and the measurement mirror 4 b. Therefore, in the elevationview of FIG. 1B, the reference light 13 a and the measurement light 13 bare parallel to the laser light 11 from the light source 1, and arereflected by the reflective device 5 so as to maintain the angle. Thelight beam reflected by the reflective device 5 passes through the lenssystem 6 and the PBS 2 and is irradiated on either the reference mirror4 a or the measurement mirror 4 b.

Reference numeral 16 denotes a light receiving device (a photodetector). The light receiving device 16 receives a multiplexed light 15which has been multiplexed by being irradiated twice on each of thereference mirror 4 a and the measurement mirror 4 b. The multiplexedlight 15 enters the light receiving device 16 at the same angle as thatof the laser light 11.

Thus, the multiplexed light 15 (a primary light beam in a double passinterferometer) enters the light receiving device 16 at the same angleas that of the laser light 11. Therefore, it becomes an interferencesignal having a period of ¼λ at a maximum which repeats uniform blinkingon the entire surface of the light receiving device 16.

Next, stray light (ghost light) which is generated by an interferometerof the present embodiment will be described.

In the present embodiment, the reference light 13 a and the measurementlight 13 b that are light beams returned from the reflective device 5will be considered. After the reference light 13 a and the measurementlight 13 b enters the PBS 2, parts of the lights reflect on an ARcoating surface (an antireflective coating surface) at each ofinterfaces 21 aa and 21 bb between the PBS 2 (the prism) and the air.The ghost lights 15 a and 15 b reflected on the AR coating surface ofthe interfaces 21 aa and 21 bb, similarly to a principal light beam (thereference light and the measurement light), enter the light receivingdevice 16 to generate an interference signal.

However, in the elevation view of FIG. 1B, each of the ghost lights 15 aand 15 b enters the light receiving device 16 at an angle which is madeby tilting at an angle 2θ with respect to a direction of the multiplexedlight 15. Therefore, an interference pattern with a predetermined pitchp is generated on a light receiving area (a light receiving surface) ofthe light receiving device 16. The pitch p of the interference patternis represented by expression 1 using a wavelength λ and an angle θ.

$\begin{matrix}{p = \frac{\lambda}{\sin\; 2\theta}} & (1)\end{matrix}$

FIG. 2 is one example of an interference pattern which appears in thelight receiving area of the light receiving device 16. Both the “light”to the right side and the “dark” to the left side in FIG. 2 indicate aninterference pattern which appears in the light receiving area of thelight receiving device 16. The interference pattern which appears in thelight receiving area repeats the “light” and the “dark” shown in FIG. 2by the displacement of the measurement mirror 4 b. This period dependson the wavelength λ, and for example, when the measurement mirror 4 b isdisplaced by around 200 nm using light having the wavelength λ of 850nm, the “light” changes to the “dark”.

In the present embodiment, as shown in FIG. 2, the interference patternhaving a pitch p represented by expression 1 can be obtained. Theinterference pattern is caused by the ghost light. Therefore, if thelight receiving area of the light receiving device 16 is sufficientlylarge compared to the pitch p, an amount of the light detected by thelight receiving device 16 is averaged (an averaging effect).Accordingly, an interpolation error can be sufficiently reduced and thelinearity of the sensor displacement output can be improved.

For example, it is considered that the interference pattern of the ghostlight 15 a reflected on the interface 21 aa is generated by theintensity of around 9% compared to the interference pattern intensity ofthe principal light beam, as described in the conventional art. In thiscase, when a diameter φ of the light receiving area is 2 mm and the tiltangle θ of each of the reference mirror 4 a and the measurement mirror 4b is 0.5°, the interference pattern generated caused by the ghost lightis a stripe pattern with a pitch p of 36 μm. The rate of the signalhaving a period of ½λ which is caused by the ghost light superimposed onan electric signal is only 0.16% of a main signal. Because this is adeterioration of only ±0.04 nm as linearity, the error can be reduced tothe extent that there is no problem at all even if it is an ultra-highaccurate length measurement of sub nanometer level. The same is true forthe other interfaces 21 b, 21 bb, and 21 a. Therefore, even ifdeterioration factors in all of these four interfaces are added, aperiodic error of ±0.16 nm is only generated. Accordingly, the lengthmeasurement can be performed with extremely high accuracy.

In the present embodiment, when the light receiving area of the lightreceiving device 16 is a circle having a diameter D, the pitch p may beset so as to be shorter than the diameter D. On the other hand, sincesin 2θ is equal to or less than 1, the pitch p is not shorter than awavelength λ. Therefore, if the pitch p is set to the range representedby expression 2, an averaging effect can be improved in the presentembodiment.

$\begin{matrix}{{\lambda \leq p} = {\frac{\lambda}{\sin\; 2\;\theta} < D}} & (2)\end{matrix}$

Furthermore, when a distance x between the reference mirror 4 a and thePBS 2 is equal to a distance r between the measurement mirror 4 b andthe PBS 2, a wave optically equivalent optical path position of thereference light and the measurement light is obtained. Therefore, theoptical path length of light passing through the glass optical pathmatches the optical path length of light passing through the air.

Therefore, a stable measurement can be performed in the long termwithout being influenced by the state change of the atmosphere relatingto an air refractive index such as atmospheric pressure, temperature, orhumidity.

When the amplitude is averaged in a state where the diameter D of thelight receiving area is equal to or more than 0.5 mm, the wavelength ofthe light source is 850 nm, and the number of the patterns in the lightreceiving surface is equal to or more than 0.5, it is preferable thatthe tilt angle θ of each of the reference mirror 4 a and the measurementmirror 4 b in the present embodiment is set to be equal to or more than0.1°. In a conventional interferometer which was made so as not to tilta reference mirror or a measurement mirror, it is predicted that theerror of the tilt angle is equal to or less than 0.1° and the effect ofthe present invention can not be obtained.

The light source apparatus (the light source 1) in the presentembodiment is an apparatus which generates collimated light, but is notlimited to this. For example, even when a divergent light source is usedinstead of the light source generating the collimated light, the sameeffect as that of the light source 1 can be obtained by using a lenssystem as a collimate unit which collimates a divergent light beam. Inthis case, the same effect as that of the light source 1 can be obtainedwith a simple configuration by integrating the lens system as thecollimate unit and the lens system 6, i.e. by constituting the lenssystem 6 as a part of the light source apparatus. In this case, thelight from the divergent light source is changed to collimated light bythe lens system 6 and enters the PBS 2.

According to the interferometer of the present embodiment, the referencemirror and/or the measurement mirror is set so as to be always tilted.Therefore, the ghost lights which have been reflected once on thereference mirror and once on the measurement mirror are multiplexed atan angle different from that of the light beams which have beenreflected twice on the reference mirror and twice on the measurementmirror. Therefore, a lot of interference patterns (interference stripes)appear and are averaged on the light receiving device, and the erroramount is considerably improved. Furthermore, the primary light beamswhich have been reflected twice on the reference mirror and twice on themeasurement mirror are multiplexed at the same angle each other.Therefore, a uniform interference state is formed and the maximuminterference intensity can be obtained.

Therefore, according to the present embodiment, a high-accuracyinterferometer which suppresses the influence of ghost light can beprovided.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions. For example, a Michelson interferometer has been described inthe present embodiment, but is not limited to this. The presentembodiment is also applicable to other interferometers such as a Fizeauinterferometer.

This application claims the benefit of Japanese Patent Application No.2008-125655, filed on May 13, 2008, which is hereby incorporated byreference herein in its entirety.

1. An interferometer comprising: a light splitting device configured tosplit light from a light source into a reference light and a measurementlight; a reference mirror configured to reflect the reference lightwhich has been split by the light splitting device and enters thereference mirror from a first direction; a measurement mirror configuredto reflect the measurement light which has been split by the lightsplitting device and enters the measurement mirror from a seconddirection; an optical system configured so that reflected lights fromthe reference mirror and the measurement mirror enter the optical systemvia the light splitting device; a reflective device configured toreflect lights from the optical system to irradiate the reference mirrorand the measurement mirror with the reflected lights through the opticalsystem, respectively; and a light receiving device configured to receivelight which have been formed by interference between two lightsreflected twice at the reference mirror and the measurement mirror,respectively, wherein the reference mirror and the measurement mirrorare tilted so that normal directions of the reference mirror and themeasurement mirror differ from the first and the second directions,respectively, and the measurement mirror is configured to be displacedalong the second direction so that the reference mirror is in apositional relation with the measurement mirror, the optical systemhaving an optical power, other than zero, that causes, in the positionalrelation, the reflected lights from the reference mirror and themeasurement mirror, passed therethrough, to be parallel with each other,and the reflective device being configured to reflect the lights, in thepositional relation, backward on respective paths, on which the lightshave respectively traveled from the optical system.
 2. An interferometeraccording to claim 1, wherein the optical system is configured todeflects the reflected lights from the reference mirror and themeasurement mirror to a direction parallel to a direction of the lightfrom the light source.
 3. An interferometer according to claim 1,wherein the light source is configured to emit a collimated light.
 4. Aninterferometer according to claim 1, wherein the reference mirror andthe measurement mirror are tilted so that passing positions of thereflected lights in the optical system are symmetric with each otherwith respect to a central axis of the optical system.
 5. Aninterferometer according to claim 1, wherein the interferometer isconfigured so that a pitch of an interference pattern which appears in alight receiving area of the light receiving device is smaller than adiameter of the light receiving area.
 6. An interferometer according toclaim 1, wherein the optical system includes one of a refractive elementhaving the optical power and a reflective element having the opticalpower.
 7. An interferometer according to claim 1, wherein the opticalsystem has a positive optical power as the optical power.